Vehicles have been developed to perform an idle-stop when idle-stop conditions are met and automatically restart the engine when restart conditions are met. Such idle-stop systems enable fuel savings, reduction in exhaust emissions, reduction in noise, and the like. As such, a number of methods may be used to control the hydraulically operated transmission to improve idle-stops and subsequent restarts, when restart conditions are met.
In one example approach, as illustrated by Nakamori et al. in U.S. Pat. No. 6,647,326, the hydraulic pressure and flow of transmission fluid (such as engine oil) to the various clutches and gears of the transmission may be maintained using a mechanical oil pump and an auxiliary electric oil pump. Specifically herein, hydraulic line pressure is maintained by the mechanical oil pump under engine running conditions and by the auxiliary electric oil pump when the engine is stopped. That is, the electric pump is operated only under engine-off conditions to circulate transmission fluid through the transmission.
Another issue often encountered in engine starting relates to engine friction. A lower temperature of the transmission fluid, for example at engine cold start, may lead to frictional losses in the transmission, largely due to an increase in the viscosity of the fluid. Similarly, engine and differential spin losses may also be incurred due to high viscosity cold lubricating fluids. These losses, in turn, may degrade fuel economy. Accordingly, various approaches have been developed to heat engine lubricating fluids at engine cold start. One example approach is shown by Tanizawa et al. in U.S. Pat. No. 6,511,396. Herein, a heat exchanger provided in the transmission hydraulic fluid loop enables heat exchange between the transmission fluid and the engine coolant, to thereby expedite heating of the transmission fluid.
However, the inventors have recognized an interrelationship between engine friction reduction via waste heat in the context of idle stop/start vehicles where hydraulic pressure is maintained during engine shut-down.
In one example, a method is provided for heating transmission fluid in a vehicle, the vehicle comprising an engine and a transmission, the engine including a heat exchanger. The method comprises, during a first engine running condition, operating an auxiliary transmission fluid pump to generate pressure in the transmission fluid and pump transmission fluid to the heat exchanger, and during a second condition where the engine is not running (e.g., shut-down at rest), operating the auxiliary transmission fluid pump to generate pressure in the transmission fluid and pump transmission fluid to the heat exchanger.
As such, transmission fluid can be circulated at least by the auxiliary pump to receive waste engine heat during engine operating conditions, such as when the transmission fluid temperature is lower than the exhaust temperature. Thus, under these conditions, transmission fluid circulating through an auxiliary fluid loop may advantageously be more rapidly heated to reduce friction. Additionally, during idle stop conditions, the auxiliary pump can again be used to generate pressure for actuation of the transmission clutches (e.g., for improved launch control), as well as circulate fluid through the auxiliary fluid loop; yet, since the engine is not operating, overheating of the transmission fluid may be reduced (as there little, if any, waste heat other than any residual heat from a previous operation). In this way, use of the auxiliary pump may be synergistically applied to both transmission fluid initial heating and maintenance of clutch operation during idle-stop engine shutdown.
An alternative implementation routes the transmission fluid through the engine coolant (instead of, or in addition to, through the engine exhaust). In this way, the engine coolant loop itself may benefit from exhaust heat recovery.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.